Retatrutide: Triple Hormone Agonism and Record Weight Reduction in Phase II
Study Overview
| Primary Reference | Jastreboff AM, et al. "Triple-Hormone-Receptor Agonist Retatrutide for Obesity." N Engl J Med. 2023;389(6):514–526. PMID: 37358530 |
| Study Design | Phase II, randomized, double-blind, placebo-controlled, dose-ranging |
| N (enrolled) | 338 adults (BMI ≥27 with ≥1 weight-related comorbidity, or BMI ≥30) |
| Doses Tested | 1, 4, 8, or 12 mg SC once weekly; vs. placebo |
| Duration | 48 weeks (with dose escalation over 16 weeks) |
| Primary Endpoint | % change in body weight from baseline to week 48 |
| Key Finding | Retatrutide 12 mg produced mean -22.8% body weight reduction at 48 weeks — the largest weight loss result published in any pharmacological obesity trial to date at time of publication |
Background
Retatrutide (LY3437943) was developed by Eli Lilly as a next-generation obesity agent targeting three incretin/metabolic receptors simultaneously: GLP-1R (glucagon-like peptide-1 receptor), GIPR (glucose-dependent insulinotropic polypeptide receptor), and GCGR (glucagon receptor). This makes it a "triple agonist," in contrast to the dual GLP-1R/GIPR agonist tirzepatide and the GLP-1R-only agonist semaglutide.
The rationale for adding glucagon receptor agonism to the dual incretin approach is based on glucagon's role in hepatic lipid mobilization and energy expenditure. Glucagon receptor stimulation increases hepatic fatty acid oxidation, reduces lipogenesis, and increases basal metabolic rate — effects that are thermogenic and lipolytic. The theoretical advantage is that the GLP-1 component suppresses appetite, the GIP component potentiates the GLP-1 effect and may reduce GLP-1 side effects, and the glucagon component drives additional energy expenditure beyond what appetite suppression alone would produce.
The Phase II trial published in the New England Journal of Medicine in July 2023 was the compound's first large-scale human efficacy study and attracted significant attention in the obesity pharmacology field for reporting weight loss percentages that exceeded all prior published pharmacological results.
Methods
Study population
Adults with BMI ≥27 kg/m² and at least one weight-related comorbidity (hypertension, dyslipidemia, obstructive sleep apnea, cardiovascular disease) or BMI ≥30 kg/m² regardless of comorbidity. Exclusion criteria included type 1 or type 2 diabetes, recent bariatric surgery, and use of weight loss medications in the prior 90 days. Mean baseline BMI was approximately 37 kg/m²; mean baseline weight approximately 108 kg.
Dose escalation and administration
All active arms began at 2 mg weekly and escalated over 16 weeks to their target dose (1, 4, 8, or 12 mg). The escalation protocol was designed to mitigate GI adverse events during dose titration. After reaching target dose, patients continued for an additional 32 weeks (total 48 weeks). SC injection was self-administered weekly.
Primary and secondary endpoints
The primary endpoint was percent change in body weight at week 48 by MMRM (mixed-model repeated measures). Secondary endpoints included proportion of participants achieving ≥5%, ≥10%, ≥15%, ≥20%, and ≥25% weight loss; waist circumference; HbA1c; fasting lipids; blood pressure; and patient-reported outcomes. A subset of participants underwent DEXA body composition analysis.
Results
Primary endpoint — body weight change at week 48
| Group | Mean % Body Weight Change | Mean Absolute Weight Loss (kg) | vs. Placebo (difference) |
|---|---|---|---|
| Placebo | -2.1% | -2.3 kg | — |
| Retatrutide 1 mg | -7.2% | -7.9 kg | -5.1% (p < 0.001) |
| Retatrutide 4 mg | -17.3% | -18.6 kg | -15.2% (p < 0.001) |
| Retatrutide 8 mg | -19.8% | -21.2 kg | -17.7% (p < 0.001) |
| Retatrutide 12 mg | -22.8% | -24.4 kg | -20.7% (p < 0.001) |
Responder analysis — weight loss thresholds
| Weight Loss Threshold | Placebo | Retatrutide 8 mg | Retatrutide 12 mg |
|---|---|---|---|
| ≥5% weight loss | 27% | 91% | 92% |
| ≥10% weight loss | 10% | 75% | 83% |
| ≥15% weight loss | 3% | 60% | 69% |
| ≥20% weight loss | 0% | 38% | 48% |
| ≥25% weight loss | 0% | 15% | 26% |
Cardiometabolic secondary endpoints (12 mg group)
| Endpoint | Baseline | Week 48 Change | p-value vs. placebo |
|---|---|---|---|
| Waist circumference (cm) | 113.2 ± 12.4 | -18.1 cm | p < 0.001 |
| Systolic blood pressure (mmHg) | 128.4 ± 11.2 | -7.2 mmHg | p < 0.001 |
| HbA1c (%) | 5.7 ± 0.4 | -0.26% | p < 0.01 |
| Triglycerides (% change) | — | -30.1% | p < 0.001 |
| HDL-C (% change) | — | +10.4% | p < 0.01 |
Body composition (DEXA subset, 12 mg)
Among participants in the DEXA body composition substudy, the 12 mg group lost approximately 83% of total weight loss as fat mass and 17% as lean mass. Fat mass reduction: -26.3 ± 3.8%; lean mass reduction: -6.4 ± 1.9%. The lean mass loss percentage (~17% of total weight loss) is comparable to what has been observed with semaglutide and tirzepatide in similar DEXA substudies, suggesting the lean mass preservation profile is driven by the weight loss magnitude rather than by retatrutide-specific effects.
Adverse events
GI adverse events were the most common: nausea (54–58% in higher dose groups vs. 20% placebo), vomiting (28–36% vs. 8%), and diarrhea (26–31% vs. 16%). The majority were mild-to-moderate and occurred during dose escalation. Serious AEs were uncommon and not drug-related in most cases. The discontinuation rate due to AEs was 14% in the 12 mg group vs. 2% placebo — higher than semaglutide STEP-1 discontinuation rate of ~7%.
Proposed Mechanism
GLP-1R agonism — appetite suppression
GLP-1R activation in hypothalamic neurons (particularly in the arcuate and paraventricular nuclei) reduces food intake by promoting satiety signaling and suppressing appetite-stimulating (orexigenic) neuropeptide Y/AgRP pathways. GLP-1R activation also delays gastric emptying, which prolongs post-meal satiety. This is the primary weight loss mechanism shared with semaglutide and the GLP-1 component of tirzepatide.
GIPR agonism — potentiation and tolerability
GIP receptor agonism potentiates the GLP-1 component's efficacy, likely through convergent hypothalamic signaling and possible adipose tissue effects. Research with tirzepatide (GLP-1/GIP) vs. semaglutide (GLP-1 alone) suggests the GIPR component adds meaningfully to weight loss magnitude. GIP may also reduce the nausea and GI tolerability burden associated with high-dose GLP-1R agonism, though this is still debated.
GCGR agonism — thermogenesis and hepatic lipid mobilization
The glucagon receptor component represents retatrutide's distinguishing feature versus tirzepatide. Glucagon receptor activation increases hepatic fatty acid oxidation through cAMP-PKA signaling, stimulates adipose tissue lipolysis, increases resting metabolic rate via brown adipose tissue thermogenesis, and reduces hepatic triglyceride content. The net effect is increased energy expenditure independent of caloric restriction — a complementary mechanism to the intake-reducing effects of GLP-1R and GIPR agonism. The combination theoretically produces greater energy deficit (higher expenditure + lower intake) than appetite suppression alone.
Limitations
- Phase II only — no cardiovascular outcomes data: Obesity drugs are now required to demonstrate cardiovascular safety (MACE outcomes). The Phase II trial was sized for efficacy, not CV safety. Retatrutide's CV safety profile is unknown pending Phase III trials
- 48 weeks — durability unknown: Weight loss trials of 48–72 weeks consistently show that the weight loss trajectory is still declining at study end for the highest doses, meaning the ultimate magnitude of weight loss (and long-term maintenance) is unknown. Discontinuation data from similar compounds suggest substantial weight regain after cessation
- No T2DM subgroup data: The Phase II trial excluded patients with diagnosed T2DM. A separate Phase II trial in T2DM patients was conducted, but the efficacy and safety profile in this population may differ from the reported results
- Lean mass loss: ~17% of total weight loss as lean mass is the current best estimate. Whether this represents clinically meaningful muscle loss — especially in older or sarcopenic patients — requires longer study with functional outcomes (strength, physical performance)
- Higher discontinuation rate than comparators: The 14% discontinuation rate from AEs in the 12 mg group suggests tolerability at the highest dose may be inferior to tirzepatide and semaglutide. Whether optimized titration schedules can reduce this remains to be demonstrated in Phase III
Research Significance
The retatrutide Phase II results represent a pharmacological inflection point in obesity medicine. The 22.8% mean weight loss at 48 weeks — with 26% of participants losing ≥25% of body weight — approaches the weight reduction magnitude achievable with bariatric surgery (typically 25–35% excess weight loss). This changes the framing of obesity pharmacotherapy from "helpful adjunct" to "potentially surgery-comparable intervention."
From a mechanistic standpoint, the retatrutide results provide evidence that adding glucagon receptor agonism to GLP-1/GIP dual agonism produces meaningful additional weight loss beyond tirzepatide's ~20.9% maximum at 72 weeks. The incremental benefit of the glucagon component (~2–3% additional weight loss) is real but modest — the dominant contribution to superior results likely reflects dose effects and the synergistic interplay of three receptor pathways simultaneously.
The dose-response data across 1–12 mg is scientifically valuable for understanding the dose-weight loss relationship and the plateau at the highest doses — the weight loss curve at 12 mg showed no clear plateau at week 48, suggesting Phase III trials at extended durations or higher doses may produce even greater weight reduction.
Full Citations
- Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity. N Engl J Med. 2023;389(6):514–526. PMID: 37358530
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205–216. PMID: 35658024
- Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989–1002. PMID: 33567185
- Nauck MA, et al. GLP-1 receptor agonists in the treatment of type 2 diabetes — state-of-the-art. Mol Metab. 2021;46:101102. PMID: 33068776
- Day JW, et al. A new glucagon and GLP-1 co-agonist eliminates obesity in rodents. Nat Chem Biol. 2009;5(10):749–757. PMID: 19597507